1. Field of the Invention
This invention relates generally to air filters in forced air ventilation systems of heating and air conditioning systems and, more particularly, to a self-charging electrostatic fan-fold filter apparatus used in a return duct of such a forced air ventilation system.
2. Description of Related Art
Conventional forced air ventilation systems in heating and air conditioning systems provide an air filtering apparatus at one location proximate the air handler or blower within what is referred to as the return ducting portion of the system. Some designs have incorporated wall or return register mounted filters at the inlet ends of return ducting leading to the blower. Typically, replaceable, flat, panel-type filters have been employed in central air conditioning and heating systems for use in the home, office, and other commercial buildings. These panel-type filters are supported in rectangular frames in the air flow passageway of the system and may be employed in banks of one or more filters, as desired. Some systems provide an air filtering apparatus end of the return duct portion of the system if there is only one return or inlet to the return duct. The area of the filter material is limited by the cross-sectional area of the duct that the filter is disposed in. Usually, a large sheet of filter material is used that is either disposable or washable, the latter requiring a hosing down and, being generally difficult to wash, because of its size. Electrostatic filters made of woven material such as polypropylene for use in a furnace proximate an air handler have surface areas limited by the duct cross-sectional area. Corrugated filters, which increase available filter surface area, generally have a support material embedded with electrostatic dust attracting fibers and are relatively stiff and not washable. Examples of some of these types of electronic, electrostatic, and mechanical air filter apparatus may be seen in the Bryant pamphlet on Models 901KAX, 908C & 902B Air Cleaners referenced herein as well as U.S. Pat. No. 3,494,113 entitled "AIR FILTER ASSEMBLY AND SUB-ASSEMBLIES" by William L. Kinney.
Single filter assemblies have been proposed which provides filter media in a pleated or fan-fold arrangement forming a plurality of downstream extending pockets therein which increases the effective filter area for a single filter panel. However, problems have been encountered with the newer pleated, pocketed type filter assemblies inasmuch as the nature of the arrangement of the filter media therein in the pleated, pocketed configuration renders the assembly burdensome and cumbersome when replacement of the filter media is required. Typical pleated, pocketed filter assemblies found in the prior art included a complicated arrangement of housings, wire reinforcing grids and filter media which render very difficult the operation of withdrawing the assembly from the air flow passageway and replacing the filter media therein. To overcome these drawbacks the filter assembly disclosed in U.S. Pat. No. 3,494,113 entitled "AIR FILTER ASSEMBLY AND SUB-ASSEMBLIES" by William L. Kinney was developed a removable, preferably closed-cell foam, cartridge adapted to be positioned within a housing in sealing engagement therewith and allowing the passage of air therethrough as it passes through the passageway. It included a replaceable, preferably reinforced resin-bonded, non-woven filter media adapted to be positioned within the cartridge in a pleated arrangement forming a plurality of downstream pockets in the air flow stream and covering the entire opening in the cartridge in sealing engagement therewith. The air filter media has two superimposed continuous layers of resin-bonded, non-woven material, a plurality of rigid, generally rectangular, separate, frame members positioned between the superimposed layers of non-woven material in a spaced, side-by-side relationship for aiding in the support of the filter media in the pleated, pocketed configuration, and means securing the superimposed layers of non-woven material together around the outside periphery of each of the frame members for holding the layers together and for holding the frame members in the aforesaid positions so that the filter media may bend in the spaces between the frame members for forming the pleated, pocketed arrangement. This type of assembly is complicated and expensive to construct and expensive to use and the filter material is neither reusable or washable.